Adsorbent filter media for removal of biological contaminants in process liquids

ABSTRACT

Adsorbent filter media particularly suited for removal of biological contaminants in process liquids. A porous fixed bed of adsorbent material is formed, using only a granular adsorbent and a water-insoluble thermoplastic binder. The resulting composite filter allows for a higher amount of adsorbent with smaller adsorbent particles than conventional depth filters. Elimination of cellulose fiber, as well as the elimination of the thermoset binder, results in reduced contamination of the process liquid.

This application is a divisional of U.S. patent application Ser. No.12/718,336 filed Mar. 5, 2010 (now U.S. Pat. No. 8,403,153 issued Mar.26, 2013), which is a continuation of U.S. patent application Ser. No.11/656,184 filed Jan. 22, 2007 (now U.S. Pat. No. 7,673,757 issued Mar.9, 2010), which claims priority of provisional application Ser. No.60/774,773 filed Feb. 17, 2006, the disclosures of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

Cellulosic depth filters, such as Millistak®+ filters commerciallyavailable from Millipore Corporation, are typically used in theproduction of biopharmaceuticals, as derived from mammalian cell culturefor the purpose of clarifying various crude product fluids. Thesecomposite filters include a layer of tightly structured cellulosic depthmedia, and can be optimized to a specific application, such as retainingcolloidal particles and cell debris or retaining whole cells and largerdebris. They combine sequential grades of media in a single filtercartridge. These filters are most commonly used in polishing orsecondary clarification processes to remove small quantities ofsuspended matter from aqueous product (protein) streams. The primaryfunction of these filters is to protect or extend the service life ofmore expensive downstream separation processes, such as sterilefiltration and affinity chromatography. That is, a common applicationfor these filters is as “prefilters”, protecting downstream equipmentand media from colloidal contaminants and other cell debris. Inaddition, such depth filters are also used for the protection of viralclearance filters by removing trace quantities of agglomerated proteins.

It is also known in the industry that composite depth filters also canretain, to varying degrees, some soluble contaminants commonly found inmammalian cell cultures, such as nucleic acids, host cell proteins,lipids, surfactants, etc. This retention capability for certain solublecontaminants is based on the adsorptive properties of the depth filtermedia.

The filter media typically employed in these depth filters includesrefined cellulose fibers (wood pulp and/or cotton derived), diatomaceousearth, and a water-soluble thermoset resin binder. The diatomaceousearth (a natural form of silica containing trace amounts of varioussilicates) in these composites is typically 40-60% by weight, and isbelieved to be the essential component, adsorbing colloidal sizebiological matter such as cell fragments, organelles and agglomeratedproteins, as well as various soluble biochemicals such as proteins,lipids and nucleic acids.

However, one of the principal drawbacks of the use of these cellulosicdepth filters for the production of parenteral drugs and otherpharmaceuticals is the relatively high level of water-solublecontaminants they release into the system. Indeed, extensivepre-flushing is required to reduce the level of these organic andinorganic contaminants to acceptable levels prior to use. Furthermore,the maximum loading of diatomaceous earth adsorbent within the depthfilter media is limited to about 60% by weight, and the minimum particlesize for the adsorbent to be retained in the fiber matrix is about 10microns.

It is therefore an object of the present invention to reduce oreliminate the release of contaminants from adsorbent filters.

It is another object of the present invention to increase the content orloading of adsorbent in filter media.

It is a further object of the present invention to provide a filter withsmaller adsorbent particles in order to maximize the available surfacearea for adsorption.

Other objects and advantages of the invention will be apparent from thefollowing detailed description and the accompanying drawings.

SUMMARY OF THE INVENTION

The problems of the prior art have been overcome by the presentinvention, which provides adsorbent filter media particularly suited forremoval of biological contaminants in process liquids. A porous fixedbed of adsorbent material is formed, using only a granular adsorbent anda water-insoluble thermoplastic binder. The resulting composite filterallows for a higher amount of adsorbent with smaller adsorbent particlesthan conventional depth filters. Elimination of cellulose fiber, as wellas the elimination of the water-soluble thermoset binder, results inreduced contamination of the process liquid. As a result, extensivepre-flushing is no longer required to reduce extraneous contaminants.Improved media performance is obtained by increasing the content of theadsorbent material, and/or by using smaller adsorbent media to maximizethe available surface area for adsorption.

The resulting composite filter material is substantially or completelydevoid of cellulose and thermoset binder, and can be placed in aclarifying system downstream of a bioreactor and upstream of a sterilefilter. Other applications include pretreatment of cell culture fluidsprior to viral clearance filtration as well as chromatographicseparations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph comparing water permeability of composites of thepresent invention with conventional composites;

FIG. 2 is a graph of pre-filter differential pressure versus throughput;

FIG. 3 is a graph of sterile filter differential pressure versusthroughput; and

FIG. 4 is a chart of conductivity values of effluent passing throughvarious media.

DETAILED DESCRIPTION OF THE INVENTION

The filter media of the present invention includes an adsorbent materialand a water-insoluble thermoplastic binder. This combination can be usedto form a porous, fixed bed of adsorbent material with suitablemechanical properties for the application, including permeability,tensile strength and bending strength. The filter media is particularlyuseful as a depth filter.

Suitable adsorbent materials include diatomaceous earth, silica, porousglass, zeolites, and activated carbon. Diatomaceous earth isparticularly preferred. Additionally, chromatography media, of anyvariety of forms (beads, ground powder, etc.) and surface chemistries(ion exchange, hydrophobic, etc.), can be used as adsorbents in suchporous media. Suitable binders include thermoplastic binders such aspolyolefins, preferably polyethylene, polypropylene or mixtures thereof.The binder is preferably used in bead, powder or fiber form. By properchoice of the binder (in terms of a sufficiently high melting orsoftening point), the media can be autoclaved or otherwise steamsterilized or gamma irradiated to help reduce or eliminate anybiological contaminants therein.

The media fabrication process can depend on the binder form used. Themedia can be prepared by blending the binder with the adsorbentmaterial, followed by fusing the adsorbent particles together such as bypartially melting or softening the binder. For example, polyethylenepowder can be blended dry (such as by shaking/tumbling for severalminutes) with the adsorbent particles, such as diatomaceous earth orsilica beads, in proportions from about 1:1 to about 1:3binder:adsorbent, by weight. The resulting blended material can beplaced in a mold and heated (e.g., in a heated hydraulic press) to asuitable temperature to fuse the adsorbent particles, such as 130° C. to160° C. Diatomaceous earth and silica beads bound into 2-4 mm thick padsusing ultra-high molecular weight polyethylene powder (Mipelon™),commercially available from Mitsui Chemical, have been formed by thismethod, with an average bead diameter of 20-30 microns. As the materialheats and softens in the press, the compressive force should beperiodically adjusted to maintain a constant force during the heat cycle(about 5 to 10 minutes).

Alternatively, a wet-laid process can be used to form the media,particularly where the binder is in the form of fibers. For example,fine polyethylene fibers (Fybrel™, from Mitsui Chemical) can bedispersed in isopropanol and water, then blended into a slurry withdiatomaceous earth powder in proportions of from about 1:1 to about 1:3binder:adsorbent, by weight. The slurry is transferred to a Buchnerfunnel holding a nominal 1 micron nonwoven support material in its baseto prevent the fibers and adsorbent from passing straight through theperforations of the funnel. The bulk of the liquid is then drawn offthrough a vacuum flask. The formed disk is transferred to an oven fordrying and thermally bonding the adsorbent particles together bypartially melting or softening the polyethylene fibers.

Composite materials constructed as above exhibit very high permeability(high porosity) and low particle retention properties relative to theconventional cellulosic depth filter media conventionally available.Gravity settling of diatomaceous earth particles with polyethylene fiberor powder produces a low-density composite structure with relativelylarge void spaces.

To enhance the separation properties of the diatomaceous earth compositematerials of the present invention, the material should be compacted orcompressed prior to and/or during heating. For example, compressing thefilter media samples for about 30 seconds using a pneumatic press in therange of 100-325 psi was found to be effective. The media samples do notrelax substantially after compression; after heating to fuse thestructure into a monolith, the media maintains its compressed thicknessdimension. The composite can be compressed to about 50-70% of itsoriginal volume depending on composition and compression force.

The application of mechanical compression to the composite materialsalso can be used to regulate water permeability, which can approach thatof standard cellulosic depth filter pads. FIG. 1 illustrates waterpermeability measurements (flow rate relative to pressure differential)of composites made from Celpure™ (Advanced Minerals) diatomaceous earthand Fybrel™ (Mitsui Chemical) polyethylene microfibers, compared toconventional Millistak®+ A1HC depth filter samples. The figure showsthat the water permeabilities of the instant composites are of a rangeconsistent with that of the commercial cellulosic depth filter media.

A third granular or fibrous component may be added to theadsorbent/binder mixture as a means of manipulating the permeability ofthe product to allow for more effective use of the contained adsorbent.The additive may be a functional adsorbent or inert material but must beof a size and quantity to measurably affect the permeability of themedia. Test results have shown that affecting media permeability in thismanner does not substantially compromise (reduce) the adsorbent capacityof the media. Creating selectively larger flow channels within the mediaallows for deeper/broader penetration of the process fluid within theporous matrix that can compensate effectively for the overall loweradsorbent content of the media.

In addition to particle capture, the adsorbent porous monoliths of thepresent invention have significant advantages over conventionalcellulosic adsorbent depth filters, particularly with regard to waterextractibles. Any extractible materials are a serious concern in theproduction of parenteral pharmaceuticals. The conventional cellulosicdepth media is known to have a relatively high extractible loading thatrequires extensive flushing prior to use. The present inventors havedemonstrated that the extractibles that contribute to conductivity(inorganics) do not solely or predominantly derive from the diatomaceousearth. Indeed, the composite materials of the present invention, devoidof cellulose and thermoset binder, result in a reduction in effluentconductivity of 75-90% compared to conventional cellulosic media. Theelimination of cellulose and substitution of the thermoset water-solubleresin binder with a water-insoluble binder such as polyethylene resultin DE-based media having a drastically lower amount of inorganicextractibles. For users of such materials, there is a considerablebenefit in reducing flushing requirements with lower risk of productcontamination.

In general terms, one preferred method for preparing a preferredcomposite filter materials of the present invention is as follows. Ultrahigh molecular weight polyethylene powder, with an average particle sizeof 25 μm, and natural diatomite having a particle size range of 0.2-25μm, are dry mixed, batchwise, in a rotary V-blender that includes aninternal high-speed agitator. The mixture is transferred from theblender to a powder dispenser (or applicator). The applicator dispensesthe mixed powders onto a moving web of porous non-woven polyestermaterial at room temperature at a controlled thickness of up to 0.5inch. Multiple applications of different powder mixtures could beapplied in similar fashion to create a gradient composition adsorbentmedia.

The loose mixed powder layer is then lightly compacted and leveled bycontact with an overhead roller before being heated from below byelectrically-heated plates and simultaneously from above by IR lamps tosoften the polyethylene powder. The temperature of the heated platesescalates along the production path to a final temperature ofapproximately 340° F. The temperature of the powder mixture is held at amaximum of around 340° F. for several minutes before applying anadditional non-woven polyether web on top.

The composite material is then continuously compressed at approximately100 psi to a thickness of 0.10-0.20″ by passing through two heatedcalender rollers also set to a temperature of around 340° F. Thefinished material is then allowed to cool on a metal plate open to theair.

Example 1

Samples of various blends of diatomaceous earth and polyethylenemicrofibers fused into pads of approximately 2-4 mm in thickness weretested in a standard clarification process step related to proteinproduct recovery from mammalian cell cultures. The test was to challengethe adsorbent media with a suspension of E. coli lysate (in buffer)under constant flow conditions while monitoring the pressure rise on themedia sample (rate of plugging) as well as the quality of the effluentrelative to the volume of fluid processed. Effluent or filtrate qualitywas measured by directly filtering the fluid through a 0.2 micronsterilizing grade membrane filter, in this case Durapore® GV. Theseexperimental composite samples were again compared to Millistak®+cellulosic depth filter media.

FIGS. 2 and 3 show the pressure profiles for the adsorbent media samplesand Millistak®+ pads and the associated sterile filter profiles.Throughput is reported as the volume of fluid processed relative to thevolume of the depth filter media employed (bed volume).

As shown in the figures, the DE/PE fiber composites were capable ofmatching the tightest or most retentive grade of Millistak®+ DE media(75 grade) in both throughput and retention. The various compositesamples have rates of rise in pressure that lie at or below that of the75DE Millistak®+ media. In addition, the rates or pressure rise on thedownstream sterile filters are all nearly equivalent for the compositesof the invention as compared to 75DE Millistak®+ media, indicating acomparable level of particle retention.

Example 2

Samples of diatomaceous earth fused into a fixed-bed pad usingpolyethylene powder (Mipelon™, Mitsui Chemical) were tested for theircapacity to protect a viral retentive membrane, NFP Viresolve 180. Inthis test, the DE/PE composite (approximately 3 mm thick) was challengedwith a solution of polyclonal human IgG protein at a concentration of0.5 μm/L. The Viresolve® membrane typically yields a capacity ofapproximately 150 L/m² for this feedstock. Using the Millistak®+ A1HCdepth filter to pre-treat this feedstock for the removal of proteinagglomerates, the Viresolve membrane capacity can be increased to therange of 750-1500 L/m². Two samples of diatomaceous earth (Celpure 25and Celpure 300 (Advanced Minerals)) were blended with the Mipelon PEpowder and formed into 2-3 mm pads after heating (without compression).The DE/PE composite samples were then used to pre-treat the IgGfeedstock and the filtrate was again processed through Viresolve 180 todetermine the effect on membrane capacity.

The Celpure 25 (a fine grade DE) yielded a Viresolve capacity of 440L/m² and the Celpure 300 (a larger coarse grain DE) yielded a Viresolvecapacity of >1000 L/m². These tests indicate that a monolith ofdiatomaceous earth formed with a PE powder binder can provide the samelevel of protection for a viral retentive membrane (by the removal ofprotein agglomerates) as currently provided by cellulosic depth filterssuch as Millistak®+ depth filters.

Example 3

To gauge the cleanliness of the composite materials of the invention,material samples were flushed with clean deionized water and theconductivity of the effluent, after a prescribed flush volume, wasmeasured. Conductivity values were taken to represent the level ofsoluble metals present in the filter media. FIG. 4 shows theconductivity values obtained for various DE/PE composite samplesrelative to commercial Millistak®+ depth filter samples.

The Millistak®+ DE media is a composite of cellulose and diatomaceousearth plus a water-soluble thermoset resin binder. The CE media containsonly cellulose fiber and binder. It is evident from these measurementsthat those extractibles that contribute to conductivity (inorganics) donot derive predominantly from the diatomaceous earth. Comparing thesevalues to the DE/PE composites tested, there is reduction in effluentconductivity of 75-90%.

Example 4

10-20% of 75-100 micron porous glass beads when added to a 2:1 mixtureof powdered polyethylene (20-30 micron) and diatomite (0.5-10 micron)can reduce the hydraulic permeability of the finished media by 10-30%with no measurable loss in adsorbent capacity as measured by particlecapture and process volume.

What is claimed is:
 1. A method of forming a heat fused composite filtermedia pad comprising the steps of: a) providing an adsorbent materialselected from diatomaceous earth, silica, glass, and a zeolite, and awater-insoluble thermoplastic binder dispersed in water selected frompolyolefins, polyethylene, polypropylene, ultra-high molecular weightpolyethylene, and mixtures thereof; b) blending the adsorbent materialand the binder dispersed in water in a binder:adsorbent proportionsabout 1:1 to about 1:3 by weight; c) forming a liquid slurry from theblended adsorbent material and binder; d) filtering the slurry to drawoff the liquid; e) thermally bonding the adsorbent particles together bypartially melting or softening the binder; and f) forming a heat fusedcomposite filter media pad substantially devoid of cellulose and athermoset binder, having a thickness about 2 mm to 4 mm.
 2. The processof claim 1, wherein the adsorbent material is diatomaceous earth powderand the binder is polyethylene fibers.
 3. The process of claim 1,furthering comprising adding porous glass beads in step (b).
 4. Theprocess of claim 3, wherein the diatomaceous earth has a particle sizerange about 0.2 microns to 25 microns, and the porous glass beads havean average bead diameter about 20 microns to 30 microns.